Perovskites Very Important Paper

Molecular Intercalation and Electronic Two Dimensionality in LayeredHybrid PerovskitesTariq Sheikh, Vaibhav Nawale, Nithin Pathoor, Chinmay Phadnis, Arindam Chowdhury, andAngshuman Nag*

Abstract: In layered hybrid perovskites, such as (BA)2PbI4

(BA = C4H9NH3), electrons and holes are considered to beconfined in atomically thin two dimensional (2D) Pb–Iinorganic layers. These inorganic layers are electronicallyisolated from each other in the third dimension by theinsulating organic layers. Herein we report our experimentalfindings that suggest the presence of electronic interactionbetween the inorganic layers in some parts of the single crystals.The extent of this interaction is reversibly tuned by intercala-tion of organic and inorganic molecules in the layeredperovskite single crystals. Consequently, optical absorptionand emission properties switch reversibly with intercalation.Furthermore, increasing the distance between inorganic layersby increasing the length of the organic spacer cations system-atically decreases these electronic interactions. This finding thatthe parts of the layered hybrid perovskites are not strictlyelectronically 2D is critical for understanding the electronic,optical, and optoelectronic properties of these technologicallyimportant materials.

Introduction

Metal halide perovskites, including the layered hybridperovskites, are now considered as technologically importantoptical and optoelectronic materials.[1] One of the simplestcompositions of layered hybrid perovskites is (BA)2PbI4

(BA = C4H9NH3+).[2] It is considered that the electron and

hole are confined in atomically thin two dimensional (2D)Pb–I inorganic layers, which are electronically isolated fromeach other in the third dimension by the insulating organiclayers.[3] A millimeter thick single crystal of (BA)2PbI4

contains millions of atomically thin Pb–I semiconductinglayers (wells) in a periodic arrangement, separated byinsulating organic layers (barriers), forming a repeatingquantum well structure. Typical all-inorganic quantum wells,such as AlAs/GaAs/AlAs are well understood,[4] but the

hybrid (BA)2PbI4 quantum well is distinctly different from theall-inorganic ones. In AlAs/GaAs, the electronic properties ofindividual components AlAs and GaAs are well understood,which are then extended to understand the interface betweenthe components. But both the components of (BA)2PbI4,namely BA+ and (PbI4)

2� do not exist independently.Furthermore, the interaction between BA+ and (PbI4)

2� isionic in nature, unlike the case of AlAs/GaAs interface.Another important difference is the huge dielectric contrastbetween the organic and inorganic parts, unlike the all-inorganic quantum wells. All these complexities make itdifficult to understand the electronic and optical properties ofthe hybrid layered perovskites, demanding newer theoreticaland experimental approaches to solve the problem.[5]

There are some peculiarities in the experimentallyobserved optical and electronic data of these layered per-ovskites. For example, a single crystal of (BA)2PbI4 and otherlayered hybrid perovskite systems exhibit two (band edge)excitonic absorption and emission features, as if the samecrystal has two band gaps![6] Further, Wang et al. found thatthe edges of (BA)2PbI4 single crystals are significantly moreelectrically conducting than their terraces.[7] We hypothesizethat some of these unusual experimental observations mightarise from possible electronic interactions between the Pb–Ilayers. To verify such possibility, we intercalated insulatingmolecules in the single crystals of layered perovskites, such as(BA)2PbI4, (DA)2PbI4 (DA = decylamine), and (PEA)2SnI4

(PEA = phenylethylammonium). In addition to the increasein the distance between the Pb–I layers, the molecularintercalation disturbs the interaction between hydrocarbontails of organic cations. The photoluminescence (PL) spectrain a layered perovskite single crystal switch reversibly fromdual emission to single emission upon intercalation. Theseresults, along with the spatially resolved PL spectra suggestthat in parts of the single crystals, probably at the edges, Pb–Ilayers interact with each other. Further we tuned the strengthof this electronic interaction systematically by changing thechain length of organic spacer cations.

Results and Discussion

We synthesize (BA)2PbI4 single crystals following a re-ported method (see Supporting Information for detailedmethodology).[8] Intriguingly, PL spectrum of the as-synthe-sized single crystal shows two excitonic transitions (seeFigure S1 in the Supporting Information) similar to ourearlier report.[6] To verify whether the two emissions arise

[*] T. Sheikh, V. Nawale, Assoc. Prof. A. NagDepartment of ChemistryIndian Institute of Science Education and Research (IISER)Pune 411008 (India)E-mail: angshuman@iiserpune.ac.in

N. Pathoor, Dr. C. Phadnis, Prof. A. ChowdhuryDepartment of Chemistry, Indian Institute of Technology BombayPowai, Mumbai (India)

Supporting information and the ORCID identification number(s) forthe author(s) of this article can be found under:https://doi.org/10.1002/anie.202003509.

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How to cite:International Edition: doi.org/10.1002/anie.202003509German Editon: doi.org/10.1002/ange.202003509

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from diverse spatial locations within a crystal, we performedenergetically resolved PL imaging of (BA)2PbI4 single crystalsusing a home-built laser epi-fluorescence microscopy setup(see Supporting Information for details). Individual(BA)2PbI4 crystals can be easily visualized owing to intensegreenish-yellow hue originating from both the excitonicemissions. The PL image (Figure 1a) over both the emissionsshow that while the entire crystal is emissive, there isconsiderable spatial variation in the intensity. To selectivelydetect the spatial distribution of the lower energy emission,we collected the fluorescence image of the same crystal viaa 540 nm (ca. 2.30 eV) long-pass filter (Figure S2). Theenergy-mapped intensity images (Figure 1b and Figure S2)suggested the dominance of the lower energy (yellow)emission near the crystal edges.

PL spectral profiles recorded from several locations of thecrystal (Figure 1c) reveal a remarkable spatial variation in therelative intensities of the two transitions. We find that therelative intensity of the lower energy (2.25 eV) emission is

much higher near the edges compared to the interior regions.On the contrary, the high energy (2.39 eV) emission istypically much more pronounced far away from the crystalboundaries. Such spatial heterogeneity in PL characteristics isclearly visible in the spectrally resolved PL image (Figure 1d)obtained for a narrow strip at the bottom of the crystal.Spectral images (such as Figure 1d) additionally reveala rather gradual variation in the relative intensity of the twoexcitonic transitions upon progression from the interiorregions to the boundary zones. It is a relevant note thatstep-like edges are present throughout the surface of thesingle crystal, as evident from field emission scanning electronmicroscopy (FESEM) image (Figure 1 e) and shown sche-matically in Figure 1 f. It is likely that all these step-like edgescontribute significantly to the lower energy transition, andconsequently, the yellow emission is spread over the entirecrystal, even at locations far from the crystal boundaries.

It is to be noted here that the edge-state emission from our(BA)2PbI4 is different than the previously reported[7, 9] edge-state emission from (C4H9NH3)2(CH3NH3)2Pb3I10 and(C4H9NH3)2(CH3NH3)3Pb4I13 with (n� 2). For samples withn� 2, the small A-site cations CH3NH3

+ leads to theformation of 3D perovskite compositions at the edges.Therefore, for n� 2, formation of 3D perovskite edge-statesgives lower energy emission with peak energy similar to thatof a pure 3D perovskite.[9, 10] In our (BA)2PbI4 (n = 1), neithersmaller A-site cations are present, nor the emission energy ofedge states match with 3D perovskites.

To explore the origin of two excitonic emissions, weexposed the (BA)2PbI4 single crystals to the iodine (I2) vapors,leading to insertion (intercalation) of molecular I2 into thelayered perovskites. I2 is selected here based on the priorreport of successful intercalation of I2 into similar layeredperovskite systems.[1f] The intercalated I2 slowly comes out(de-intercalate) from the perovskite crystals, upon removingthe I2 atmosphere. This reversibility of the I2 intercalationprocess is schematically depicted in Figure 2 a. The interca-lation of I2 is expected to increase the distance between Pb–Ilayers. Indeed, the powder X-ray diffraction (PXRD) patternsof (BA)2PbI4 single crystal (Figure 2 b) show a shift in thepeak position towards the lower angles (2q values) upon I2

insertion, which indicates an increase in the interlayerdistance.[6,8] Subsequent de-intercalation process reverts thePXRD peaks to the original 2q values.

The PL spectrum of (BA)2PbI4 single crystal (Figure 2c)shows dual emission peaks as discussed above.[6, 11] Interest-ingly, after I2 intercalation, the lower energy (2.21 eV)emission completely disappears without effecting the higherenergy (2.38 eV) emission. Spatially resolved PL spectrosco-py performed after I2 intercalation validate that only thehigher energy (green) emission emanates from entire crystals,and the lower energy emission is non-existent even at thecrystal boundaries (Figure S3). Subsequently, during the de-intercalation process, the lower energy emission reappearswithin an hour, as shown in Figure 2d. Upon the progressiverelease of I2, from the (BA)2PbI4 lattice, we observeda systematic increase in the intensity of lower energy emissionwith (de-intercalation) time. The intercalation of I2 increasesthe distance between the inorganic layers, and further it is

Figure 1. a) Fluorescence intensity (false color) image of a (BA)2PbI4

single crystal, b) pseudo-colored PL intensity image generated by thesuperposition of PL images collected at two energies. The intensitiesare adjusted to reproduce nearly a true color of the crystal asvisualized through the microscope. c) Spatially resolved PL emissionspectra from the edges (1,4) and the interior regions (2,3), locationsmarked in (b)). d) Spectrally resolved PL image of a rectangular stripat the bottom of the crystal (marked with a dotted rectangular in (b)).Intensity calibration is in counts per seconds (cps). e) A representativeSEM image of the top surface of (BA)2PbI4 single crystal. f) Schematicrepresentation showing the edges present on the surface of a layeredcrystal.

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expected to disrupt the interactions between the hydrocarbontails of BA+ ions (see Figure 2a) thereby reducing thestructural rigidity across the Pb–I layers. Therefore, thereversible disappearance and reappearance of the lowerenergy emission at 2.21 eV with intercalation and de-inter-calation of I2 respectively, suggests that the lower energyemission originates from some electronic interaction betweenthe Pb–I layers.

These results suggest that there are parts in a single crystalof (BA)2PbI4 showing the high-energy PL emission, wherePb–I layers do not interact with each other, as this emission isindependent of the molecular intercalation. In some otherparts, Pb–I layers interact with each other originating thelower energy PL emission. Spatially resolved PL spectra(Figure 1) suggest that the edges of the crystals have morecontribution from the lower energy PL emission. Therefore,the edges of the crystals can be considered as those locationswhere the Pb–I layers interact predominantly, introducing thepseudo 2D nature in the system. These results agree with therecent report that showed edges could conduct electricityunlike other parts of (BA)2PbI4 single crystal.[7] It is to benoted here that electron injection/extraction, required for anyelectronic and optoelectronic applications, occur throughthese edge states because the terraces of layered perovskitesare insulating. Therefore, not only the optical properties butalso the charge transfer or transport properties need toconsider this pseudo 2D electronic nature of the edge states.

Unfortunately, there is an overlap in the optical absorp-tion of intercalated I2 with (BA)2PbI4 (see Figure S4), whichcomplicates the comparison of the absorption spectra beforeand after the intercalation process. To compare both absorp-tion and emission spectra before and after the intercalationprocess, we searched for a system where there is no overlap inthe absorption of the intercalating molecules with that of thehost perovskite. From the literature, we found that molecularhexane can be intercalated into layered perovskites. How-ever, as hexane is larger than I2, the chain length of organiccations in the hybrid perovskite also has to be longer toaccommodate hexane.[12] Consequently, (DA)2PbI4 (DA =

decylamine) single crystals are synthesized and intercalatedwith hexane. Details of synthesis, characterization andintercalation methodologies are provided in the SupportingInformation. Figure 2e depicts the reversible hexane inter-calation in a (DA)2PbI4 single crystal. PXRD patterns (Fig-ure S5) show an increase in interlayer distance after hexaneintercalation, and the interlayer distance comes back to theoriginal value after de-intercalation of hexane. PL spectra inFigure 2 f switch from dual emission for pristine (DA)2PbI4 toa single emission for hexane intercalated (DA)2PbI4. Thedisappearance of the lower energy emission is also accom-panied by a blue-shift in the absorption spectrum of(DA)2PbI4 after hexane intercalation, as shown in Figure S6.The switching of PL from dual to single emission is againreversible with intercalation/de-intercalation of hexane, as

Figure 2. a) Schematic representation of the reversible intercalation of molecular iodine into a (BA)2PbI4 single crystal. b) PXRD patterns andc) PL spectra of (BA)2PbI4 single crystal with and without I2 intercalation. d) Evolution of PL peak during the de-intercalation process of(BA)2PbI4:I2. e) Schematic representation of the reversible intercalation of molecular hexane in (DA)2PbI4 single crystal. f) PL spectra of (DA)2PbI4

single crystal before and after hexane intercalation.

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shown in Figure S7. Overall, the reversible intercalation of I2

and hexane into (BA)2PbI4 and (DA)2PbI4 single crystals,respectively, resulted in similar reversible changes in opticalabsorption and emission. In both cases, intercalations reduce(or eliminates) interactions between the Pb–I layers, elimi-nating the lower energy emission.

We preferred reversible intercalation in the previous casesfor reliable correlation of intercalation with electronicdimensionality and optical properties. However, this rever-sible process has one demerit. Structural analysis of theintercalated crystals using single-crystal XRD is challenging,because of the de-intercalation process during the measure-ment. So, it remains unclear whether the intercalatedmolecules are ordered or disordered and how many moleculesare present within the intercalating layers. To address thisissue, we made irreversible intercalation of hexafluoroben-zene (HFB) in phenylethylammonium tin iodide[(PEA)2SnI4], following the work of Mitzi et al.[13] The highlyelectron-deficient HFB strongly binds with the electron-richphenyl rings of (PEA)2SnI4, providing stability to theintercalated system that can be termed as (PEA)2SnI4DHFB.

Methodologies for synthesizing and intercalating(PEA)2SnI4 single crystals are given in the SupportingInformation. PXRD patterns provided in Figure S8 confirmthe intercalation of HFB between the (PEA)2SnI4 layers.Figure 3a shows the crystal structure of (PEA)2SnI4 beforeand after intercalation, obtained by recording single-crystalXRD. Both (PEA)2SnI4 and (PEA)2SnI4:HFB crystalize ina triclinic lattice with P(�1) space group. The latticeparameters of (PEA)2SnI4 and (PEA)2SnI4:HFB are a =

8.6321(16) �, b = 8.6344(16) �, c = 16.358(3) �, a = 94.696-(5)8, b = 100.426(5)8, g = 90.450(5)8 and a = 8.5663(12) �, b =

8.5699(12) �, c = 20.485(3) �, a = 98.638(4)8, b = 93.594(4)8,g = 90.064(4)8 respectively (see Table S1). The HFB mole-cules are sandwiched between the phenyl rings of twophenylethylammonium ions of the adjacent layers in a regularpattern. Similar to the lead halide systems, (PEA)2SnI4 singlecrystal also shows a dual emission with peak energies at1.98 eV and 1.82 eV, as shown in Figure 3b. The energydifference between the two peaks is nearly similar to thatobserved in lead halide systems. These results rule out anysignificant role of spin-orbit splitting for the occurrence ofdual emission since the spin-orbit coupling is significantlydiminished in Sn-halides compared to Pb-halides. Interest-ingly, (PEA)2SnI4:HFB shows a single PL emission (Fig-ure 3b).

Therefore, the switching of dual PL emissions to a singlePL emission after intercalation is a generic observation for allthe three systems, namely, I2 in (BA)2PbI4, hexane in(DA)2PbI4 and HFB in (PEA)2SnI4. These results confirmthat in all the three pristine layered perovskite single crystals,the lower energy emission peaks come from the parts ofcrystals where Pb–I or Sn–I layers interact with each other. Itis noteworthy here that the single emission peak in(PEA)2SnI4:HFB is slightly blue-shifted compared to thehigher energy emission of (PEA)2SnI4. The absorptionspectra (Figure S9) also show a blue-shift upon HFB inser-tion. This blue-shift in PL and absorbance is because of someminor structural difference between (PEA)2SnI4 and

(PEA)2SnI4:HFB (see Figure S10 and S11) similar to theprior report.[13] The average Sn-I-Sn bond angle decreasesfrom around 155.98 to 152.28 and average Sn–I bond lengthincreases from approximately 3.139 � to 3.152 � upon HFBintercalation in (PEA)2SnI4. Both the decreased bond anglesand increased bond lengths support the observed blue-shift inthe PL emission, upon HFB intercalation.[6]

To validate our hypothesis of the presence of theinteractions between the Pb–I layers, we also adopteda different approach where we tuned the strength ofinteractions between Pb–I layers by changing the distancebetween the Pb–I layers. This can be achieved by changing thechain length of organic cations R-NH3

+. As the length oforganic cations is systematically increased from C4

(C4H9NH3+) to C16 (C16H33NH3

+), including only even num-ber of carbon atoms, the interlayer distance and barrier widthincreased systematically, as shown in Figure 4a. The system-atic shift of PXRD peaks toward the lower 2q values, shown inFigure 4b, confirms the increased distance between Pb–Ilayers with increasing the C-chain lengths. Both high thick-ness and high absorption cross-section do not allow light with

Figure 3. a) Crystal structure of (PEA)2SnI4 before and after HFBintercalation. b) PL spectra of a (PEA)2SnI4 single crystal before andafter HFB intercalation. CCDC numbers can be found in the Support-ing Information.

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energy above the band gap to pass through the single crystals.Consequently, when we measure the UV/Vis absorptionspectra of these crystals in transmission mode (Figure 4c),only the lowest energy absorption edge is observed, beyondwhich the absorbance saturates for all the samples. Never-theless, Figure 4c clearly shows a systematic blue-shift in thelowest energy absorption edge with increasing C-chain length.The absorption spectra recorded in the reflectance mode alsoshow a similar trend (Figure S12). PL spectra in Figure 4dshow two well-separated emissions for samples with C-chainlengths C4 to C12. The lower energy emission peak, similar totheir absorption edges, systematically blue-shifts with increas-ing C-chain length. In contrast, the higher energy emission at2.38 eV remains unchanged for all the samples. For a large

enough interlayer distance in C14 system [(C14H29NH3)2PbI4],the lower energy emission overlaps with, the higher energyemission giving rise to a single but asymmetric and broaderemission. With further increase in the interlayer spacing in C16

system [(C16H33NH3)2PbI4], a single emission peak withnarrower spectral width is observed.

These results corroborate that the higher energy emissionat 2.38 eV does not depend on barrier width, (i.e., the C-chainlength), similar to the intercalation results in Figure 2.Therefore, this higher energy peak is attributed to individual2D layers and is independent of interlayer interaction,whereas, the lower energy emission peak depends on barrierwidth. Lower the barrier width with shorter carbon chain, thestronger is the interlayer electronic interaction, and therefore,

Figure 4. a) Schematic representation showing an increase in interlayer distance by increasing the chain length of R-NH3+ spacer cation. The

colors are indicative of the actual colors of the synthesized crystals at room temperature. b) PXRD patterns, c) UV/Vis absorption spectra intransmission mode, and d) PL spectra of layered lead iodide perovskites with spacer cation varying from butylammonium (C4) tohexadecylammonium (C16). Layered perovskites exist in different phases at different temperatures. Note that the samples C4 to C10 transform tothe low-temperature phase below room temperature, whereas for C12 to C16 samples, the phase transition happens above the room temperature(310–340 K).[14, 15] So at room temperature, C12 to C16 samples have a different phase compared to C4 to C10 samples. To compare the optical datain the same phase, we recorded the optical data for samples C4 to C10 at room temperature, whereas for samples C12 to C16, optical data arerecorded at 333–343 K. More details of phase transitions and optical data are discussed in the Supporting Information (Figure S13–S15). Thesymbol “*” in (c) and (d) indicates that the data recorded after heating the sample to 333–343 K.

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more is the red-shift compared to the higher energy emissionpeak. For samples like C12, we still observe the signature ofsmall interlayer interactions. The crystal structure suggestsa distance of approximately 1.7 nm between Pb–I layers.[14]

This is a large distance for possible electronic interactions.However, one would expect a structural reorganization at theedges, probably reducing the Pb–I inter-layer distance facil-itating the inter-layer electronic interactions.[7, 9] This could bea plausible reason behind the higher relative intensity oflower energy PL near the edges, as shown in Figure 1. Furthertheoretical and experimental studies are required to under-stand the structural and compositional characteristics of edgesof these layered perovskites.[9]

Conclusion

Two excitonic emissions from single crystals of layeredhybrid lead and tin halide perovskites are puzzling. Spatiallyresolved fluorescence microscopy imaging of (BA)2PbI4

single crystals shows that the ratio of the two emission peakschange at different parts of the crystal, with the higherrelative intensity of lower energy emission near the edges ofthe crystal. Interestingly, intercalation of iodine in (BA)2PbI4

and hexane in (DA)2PbI4 reversibly switches the PL from twoemission peaks to a single emission peak, by the disappear-ance of the lower energy PL peak. Similarly, insertion of HFBin (PEA)2SnI4 single crystal also eliminates the lower energyPL peak. Therefore, we assign the lower energy emission peakto Pb–I interlayer interaction. Furthermore, we decrease thestrength of the interlayer interaction almost continuously byincreasing the chain length (C4 to C16) of R-NH3

+ spacercations. Subsequently, the lower energy PL peak systemati-cally blue-shift with increasing chain length from C4 to C12,and eventually merge with high energy peak for C14 and C16.All these results show that in parts (particularly edges) of thelayered hybrid perovskites crystals, the Pb–I inorganic layerselectronically interact with each other. This finding not onlyexplains the unusual optical properties of these systems butwill also be important to understand and design chargetransfer and charge transport pathways in optoelectronicdevices of layered perovskite.

Acknowledgements

A.N. acknowledges the Science & Engineering ResearchBoard (SERB, EMR/2017/001397) India. A.C. acknowledgesfinancial support (SERB Grant No. EMR/2017/004878) fromthe DST (Govt. of India) and MNRE (Govt. of India) aidedNCPRE at IIT Bombay. T.S. acknowledges the UniversityGrants Commission (UGC) India for research fellowship. C.P.thanks IIT Bombay for institute postdoctoral fellowship andN.P. acknowledges CSIR India for research fellowship.

Conflict of interest

The authors declare no conflict of interest.

Keywords: intercalation · layered perovskites ·optical properties · perovskites · 2D semiconductors

[1] a) M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J.Snaith, Science 2012, 338, 643 – 647; b) L. Protesescu, S. Yakunin,M. I. Bodnarchuk, F. Krieg, R. Caputo, C. H. Hendon, R. X.Yang, A. Walsh, M. V. Kovalenko, Nano Lett. 2015, 15, 3692 –3696; c) A. Swarnkar, R. Chulliyil, V. K. Ravi, M. Irfanullah, A.Chowdhury, A. Nag, Angew. Chem. Int. Ed. 2015, 54, 15424 –15428; Angew. Chem. 2015, 127, 15644 – 15648; d) S. Das Adhi-kari, S. K. Dutta, A. Dutta, A. K. Guria, N. Pradhan, Angew.Chem. Int. Ed. 2017, 56, 8746 – 8750; Angew. Chem. 2017, 129,8872 – 8876; e) J. Pal, S. Manna, A. Mondal, S. Das, K. V. Adarsh,A. Nag, Angew. Chem. Int. Ed. 2017, 56, 14187 – 14191; Angew.Chem. 2017, 129, 14375 – 14379; f) M. D. Smith, L. Pedesseau, M.Kepenekian, I. C. Smith, C. Katan, J. Even, H. I. Karunadasa,Chem. Sci. 2017, 8, 1960 – 1968; g) Q. A. Akkerman, G. Rain�,M. V. Kovalenko, L. Manna, Nat. Mater. 2018, 17, 394 – 405;h) N. Zhou, Y. Shen, L. Li, S. Tan, N. Liu, G. Zheng, Q. Chen,H. J. Zhou, J. Am. Chem. Soc. 2018, 140, 459 – 465; i) L. Mao, W.Ke, L. Pedesseau, Y. Wu, C. Katan, J. Even, M. R. Wasielewski,C. C. Stoumpos, M. G. Kanatzidis, J. Am. Chem. Soc. 2018, 140,3775 – 3783; j) P. Pal, S. Saha, A. Banik, A. Sarkar, K. Biswas,Chem. Eur. J. 2018, 24, 1811 – 1815; k) C. Katan, N. Mercier, J.Even, Chem. Rev. 2019, 119, 3140 – 3192; l) M. Li, J. Zhou, G.Zhou, M. S. Molokeev, J. Zhao, V. Morad, M. V. Kovalenko, Z.Xia, Angew. Chem. Int. Ed. 2019, 58, 18670 – 18675; Angew.Chem. 2019, 131, 18843 – 18848; m) L. Zhou, J.-F. Liao, Z.-G.Huang, J.-H. Wei, X.-D. Wang, W.-G. Li, H.-Y. Chen, D.-B.Kuang, C.-Y. Su, Angew. Chem. Int. Ed. 2019, 58, 5277 – 5281;Angew. Chem. 2019, 131, 5331 – 5335; n) R. Zhang, X. Mao, Y.Yang, S. Yang, W. Zhao, T. Wumaier, D. Wei, W. Deng, K. Han,Angew. Chem. Int. Ed. 2019, 58, 2725 – 2729; Angew. Chem.2019, 131, 2751 – 2755; o) L. Zhou, J.-F. Liao, Z.-G. Huang, J.-H.Wei, X.-D. Wang, H.-Y. Chen, D.-B. Kuang, Angew. Chem. Int.Ed. 2019, 58, 15435 – 15440; Angew. Chem. 2019, 131, 15581 –15586.

[2] a) D. H. Cao, C. C. Stoumpos, O. K. Farha, J. T. Hupp, M. G.Kanatzidis, J. Am. Chem. Soc. 2015, 137, 7843 – 7850; b) W. Peng,J. Yin, K.-T. Ho, O. Ouellette, M. De Bastiani, B. Murali, O.El Tall, C. Shen, X. Miao, J. Pan, E. Alarousu, J.-H. He, B. S. Ooi,O. F. Mohammed, E. Sargent, O. M. Bakr, Nano Lett. 2017, 17,4759 – 4767; c) K. Leng, I. Abdelwahab, I. Verzhbitskiy, M.Telychko, L. Chu, W. Fu, X. Chi, N. Guo, Z. Chen, Z. Chen, C.Zhang, Q.-H. Xu, J. Lu, M. Chhowalla, G. Eda, K. P. Loh, Nat.Mater. 2018, 17, 908 – 914.

[3] a) D. B. Mitzi, C. A. Feild, W. T. A. Harrison, A. M. Guloy,Nature 1994, 369, 467 – 469; b) D. B. Mitzi, S. Wang, C. A. Feild,C. A. Chess, A. M. Guloy, Science 1995, 267, 1473 – 1476; c) J.Even, L. Pedesseau, C. Katan, ChemPhysChem 2014, 15, 3733 –3741.

[4] a) D. Gammon, B. V. Shanabrook, D. S. Katzer, Appl. Phys. Lett.1990, 57, 2710 – 2712; b) C. Livache, B. Martinez, N. Goubet, C.Gr�boval, J. Qu, A. Chu, S. Royer, S. Ithurria, M. G. Silly, B.Dubertret, E. Lhuillier, Nat. Commun. 2019, 10, 2125.

[5] B. Traore, L. Pedesseau, L. Assam, X. Che, J.-C. Blancon, H.Tsai, W. Nie, C. C. Stoumpos, M. G. Kanatzidis, S. Tretiak, A. D.

AngewandteChemieResearch Articles

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Mohite, J. Even, M. Kepenekian, C. Katan, ACS Nano 2018, 12,3321 – 3332.

[6] a) T. Sheikh, A. Shinde, S. Mahamuni, A. Nag, ACS Energy Lett.2018, 3, 2940 – 2946; b) T. Sheikh, A. Nag, J. Phys. Chem. C 2019,123, 9420 – 9427.

[7] K. Wang, C. Wu, Y. Jiang, D. Yang, K. Wang, S. Priya, Sci. Adv.2019, 5, eaau3241.

[8] C. C. Stoumpos, D. H. Cao, D. J. Clark, J. Young, J. M. Rondi-nelli, J. I. Jang, J. T. Hupp, M. G. Kanatzidis, Chem. Mater. 2016,28, 2852 – 2867.

[9] a) J.-C. Blancon, H. Tsai, W. Nie, C. C. Stoumpos, L. Pedesseau,C. Katan, M. Kepenekian, C. M. M. Soe, K. Appavoo, M. Y.Sfeir, S. Tretiak, P. M. Ajayan, M. G. Kanatzidis, J. Even, J. J.Crochet, A. D. Mohite, Science 2017, 355, 1288 – 1292; b) E. Shi,S. Deng, B. Yuan, Y. Gao, Akriti, L. Yuan, C. S. Davis, D.Zemlyanov, Y. Yu, L. Huang, L. Dou, ACS Nano 2019, 13, 1635 –1644.

[10] L. Na Quan, D. Ma, Y. Zhao, O. Voznyy, H. Yuan, E. Bladt, J.Pan, F. P. Garc�a de Arquer, R. Sabatini, Z. Piontkowski, A.-H.

Emwas, P. Todorovic, R. Quintero-Bermudez, G. Walters, J. Z.Fan, M. Liu, H. Tan, M. I. Saidaminov, L. Gao, Y. Li, D. H.Anjum, N. Wei, J. Tang, D. W. McCamant, M. B. J. Roeffaers, S.Bals, J. Hofkens, O. M. Bakr, Z.-H. Lu, E. H. Sargent, Nat.Commun. 2020, 11, 170.

[11] D. B. Mitzi, Chem. Mater. 1996, 8, 791 – 800.[12] Y. I. Dolzhenko, I. Tamotsu, M. Yusei, Bull. Chem. Soc. Jpn.

1986, 59, 563 – 567.[13] D. B. Mitzi, D. R. Medeiros, P. R. L. Malenfant, Inorg. Chem.

2002, 41, 2134 – 2145.[14] D. G. Billing, A. Lemmerer, New J. Chem. 2008, 32, 1736 – 1746.[15] a) D. G. Billing, A. Lemmerer, Acta Crystallogr. Sect. B 2007, 63,

735 – 747; b) A. Lemmerer, D. G. Billing, Dalton Trans. 2012, 41,1146 – 1157.

Manuscript received: March 7, 2020Accepted manuscript online: April 3, 2020Version of record online: && &&, &&&&

AngewandteChemieResearch Articles

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Research Articles

Perovskites

T. Sheikh, V. Nawale, N. Pathoor,C. Phadnis, A. Chowdhury,A. Nag* &&&&—&&&&

Molecular Intercalation and ElectronicTwo Dimensionality in Layered HybridPerovskites

Trial separation : 2D layered Pb- and Sn-halide perovskites show surprising opti-cal properties, such as dual excitonicemissions. Such unusual properties areshown to arise through to the interactionbetween the 2D inorganic layers. Sepa-rating the inorganic layers either bymolecular intercalation or by increasingthe length organic spacer ion reversiblyswitches the optical properties.

AngewandteChemieResearch Articles

&&&&Angew. Chem. Int. Ed. 2020, 59, 2 – 9 � 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

These are not the final page numbers! � �